Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly, onto an image substrate. For example, the printhead assembly and the image substrate are moved relative to one other and the inkjets are operated to eject ink drops onto the image substrate at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or a series of print medium sheets onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form that is delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink that is supplied to a print head.
During the operational life of these imaging devices, inkjets in one or more printheads may become unable to eject ink in response to a firing signal. These inoperative inkjets are also called malfunctioning inkjets or ejectors. The defective condition of the inkjet may be temporary and the inkjet may return to operational status after one or more image printing cycles. In other cases, the inkjet may not be able to eject ink until a purge cycle is performed. A purge cycle may successfully unclog inkjets so they are able to eject ink once again. Execution of a purge cycle, however, requires the imaging device to be taken out of its image generating mode. Thus, purge cycles affect the throughput rate of an imaging device and are preferably performed during periods in which the imaging device is not generating images. Also, the purge cycle may not successfully unclog all inkjets and the printing device may need to function with some number of malfunctioning inkjets until the printhead is replaced.
Digital three-dimensional manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object of virtually any shape from a digital model. Three-dimensional printing is an additive process in which one or more ejector heads eject successive layers of material on a substrate in different shapes in a manner similar to the operation of ejector heads in an inkjet printer. Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling. During printing of an object, one or more ejectors can deteriorate by ejecting the material at an angle, rather than normal, to the ejector, ejecting drops that are smaller than an ejector should eject, or by failing to eject any drop at all. An ejector suffering from any of these operational deficiencies is known as a malfunctioning ejector. If the operational status of one or more ejectors deteriorates during object printing, the quality of the printed object may not be assessed until the printing operation is completed. Consequently, object jobs requiring many hours or multiple days can produce objects that do not conform to specifications due to malfunctioning ejectors in the ejector heads. Once such objects are detected, the printed objects are scrapped, restorative procedures are applied to the ejector heads to restore ejector functionality, and the print job is repeated.
Methods have been developed that enable an imaging device to generate images or object layers even though one or more inkjets or ejectors in the imaging device or three-dimensional object printer are unable to eject ink or material. These methods cooperate with image rendering methods to control the generation of firing signals for inkjets in a printhead. Rendering refers to the processes that receive input image data values and then generate output image values. The output image values are used to generate firing signals for printheads to cause the inkjets to eject ink onto the recording media. Once the output image values are generated, a method may use information regarding defective inkjets detected in a printhead to identify the output image values that correspond to a defective inkjet in a printhead. The method then searches to find a neighboring or nearby output image value that can be adjusted to compensate for the defective inkjet. Preferably, an increase in the amount of ink ejected near the defective inkjet may be achieved by replacing a zero or nearly zero output image value with the output image value that corresponds to the defective inkjet. Another method increases neighboring or nearby output image values to boost the amount of ink to be ejected by a plurality of inkjets in the vicinity of the defective inkjet. Another method is able to compensate for the defective inkjet because a normalization process may be used to establish a maximum output image value for inkjets that is less than the output value that causes an inkjet to eject the maximum amount of ink that can be ejected by an inkjet. Thus, an output image value can be increased beyond the normalized maximum output image value to enable an inkjet to eject an amount of ink corresponding to the maximum output value plus some incremental amount. By firing several nearby inkjets in this manner, the ejected ink density can approximate the ink mass that would have been ejected had the defective inkjet been able to eject the ink for a missing pixel. Another method may rely on multiple inkjets to print the image levels within a pixel. When one of those ejectors malfunctions some of the ink can be printed with drops from one of the other functioning ejectors within that same pixel location.
Ejector heads have been developed that have inkjets and ejectors that eject multiple drops of ink or material at a single location or that eject drops of different masses or volumes at a location. To represent the multiple sizes or drops at a location in an image data array, the rendering process converts continuous tone, sometimes called contone data, to multi-level tone data. “Multi-level tone data” refers to output data that has more than two values, but fewer values than the permissible range for the contone data. For example, grayscale image data is continuous tone data having a range of 0 to 255. The data in this range of grayscale data can be converted to multi-level tone data having a 0, 1 or 2 to represent no drop, a first volume drop or a second volume drop, respectively. The conversion can be performed by comparing the grayscale value for a pixel to two thresholds of different values. If the grayscale value is less than or equal to the lowest threshold then the multi-level tone data value is zero, if the grayscale value is between the two thresholds then the multi-level tone data value is one, and if the grayscale value is greater than or equal to the highest threshold then the multi-level tone data value is two. Developing a compensation scheme for multi-level tone data corresponding to a malfunctioning inkjet or ejector would be useful.